In conventional technology carbon monoxide is obtained during the steam reforming or partial oxidation of hydrocarbons. Carbon monoxide of high purity can be produced in such units by employing conventional purification methods. Cryogenic purification methods are known, such as partial condensation or scrubbing with liquid methane; other processes employed for the purification are absorption processes such as the Tenneco Chemicals Cosorb, or adsorption processes (Pressure Swing Adsorption or PSA).
Carbon monoxide is also produced during the preparation of the synthesis mixture employed for ammonia synthesis, when hydrogen is obtained by partial oxidation of hydrocarbons or of coal. Such a process for the preparation of the synthesis mixture illustrated in the diagram of FIG. 1 and will be described in outline below.
After the removal of soot, in 1, and that of hydrogen sulphide, in 2, from the mixture originating from the stage of partial oxidation of hydrocarbons or of coal, in 3, stage 4 of CO conversion converts most of the CO present, under the effect of steam, into carbon dioxide (CO.sub.2), producing hydrogen. After the removal of CO.sub.2 by absorption, in 5, the gas mixture is subjected to a stage 6 of scrubbing with nitrogen, which yields the synthesis mixture (N.sub.2 +3H.sub.2) and a tail gas containing essentially CO, H.sub.2, methane CH.sub.4 and N.sub.2.
Also shown in FIG. 1 are the other operations of ammonia synthesis: air distillation, at 7, yielding the oxygen needed for the partial oxidation 3 and the nitrogen needed for the scrubbing 6; actual synthesis of NH.sub.3 in 8, from the mixture originating from stage 6, compressed at 9; vapour decompression in a turbine 10 to drive the compressor 9; treatment of the hydrogen sulphide originating from stage 2, in 11, by a Claus process; and various heat exchangers 12 to 14.
In the prior art this tail gas from the stage of scrubbing with nitrogen was burned.
A process for the production of CO which, in the particular case above, enables the CO-rich residual gas to be reused profitably as a mixture for feeding a process for the production of CO is known from EP-A-0,092,770. One of the processes described in this patent application comprises a stage of scrubbing in methane in order to separate hydrogen from a mixture also including methane, carbon monoxide and nitrogen. The condensate thus produced is distilled in a first distillation column to separate methane from the remainder of the mixture. The gas from the head of this first column, containing chiefly carbon monoxide is distilled in a second distillation column to remove nitrogen and the remaining hydrogen, pure carbon monoxide being drawn from the sump of this column.
The carbon monoxide is subsequently used as refrigerant for the condensers at the head of the first and of the second column.